1. Field of the Invention
The present invention relates to a non-volatile memory device including a memory cell having volatile and non-volatile data storage capacity. More particularly, the present invention relates to a Non-Volatile Dynamic Random Access Memory (hereinafter referred to as xe2x80x9cNVDRAMxe2x80x9d) including a memory cell having a combination of a Dynamic Random Access Memory (hereinafter referred to as xe2x80x9cDRAMxe2x80x9d) as a volatile data storage means and a Electrically Erasable and Programmable Read Only Memory (hereinafter referred to as xe2x80x9cEEPROMxe2x80x9d) as a non-volatile data storage means, or a memory cell having volatile and non-volatile data storage capacity using a ferroelectric material. Further, the present invention relates to a system Large-Scale Integrated circuit (hereinafter referred to as xe2x80x9cLSIxe2x80x9d) such as a system control circuit or system solution chip; a system Center Processing Unit (hereinafter referred to as xe2x80x9cCPUxe2x80x9d); a system Multi Processing Unit (hereinafter referred to as xe2x80x9cMPUxe2x80x9d); and the like.
2. Description of the Related Art
Conventional NVDRAMs are typically categorized into two types. One type includes a combination of a DRAM and a EEPROM. The other type employs a ferroelectric material.
The structure and operation of the DRAM and EEPROM combination type NVDRAMs are described in xe2x80x9cA 256 K-bit Non-Volatile PSRAM with Page Recall and Chip Storexe2x80x9d, 1991 Sym. VLSI Circuit Dig. Tech. Papers, May, pp. 91-92, and in xe2x80x9cDevelopment of 256 K bit Non-Volatile DRAM (NV-DRAM) Operating as a Pseudo-SRAMRxe2x80x9d, Sharp Technical Journal, No. 49, pp. 45-49, June, 1991.
In a normal volatile (DRAM) operation, the DRAM and EPPROM combination type NVDRAM uses its DRAM for data access. In a store operation, immediately before the NVDRAM is turned off or at any time desired, data held in the DRAM is transferred to the EEPROM. In the recall operation, the data stored in the non-volatile state is transferred from the EEPROM to the DRAM.
As a result, a high-speed data access can be achieved by the DRAM in the normal DRAM operation while the data can be saved safely in the EEPROM in the absence of supplied power. The advantages of the combination type NVDRAM are that data can be retained separately in the DRAM and the EEPROM; and that there is no limitation to the number of times the recall operation can be repeated.
On the other hand, a ferroelectric type NVDRAM is described in (1) xe2x80x9cAn Experimental 512-bit Non-Volatile Memory with Ferroelectric Storage cellxe2x80x9d, IEEE Journal of Solid State Circuites, Vol. 23, pp. 1171-1175, October, 1988; (2) xe2x80x9cA Ferroelectric DRAM Cell for High-Density NVRAM""sxe2x80x9d, IEEE Electron Device Lett. Vol. 11, No. 10, pp. 454-456, October, 1990; and (3) Japanese Laid-Open Publication No. 4-42498, xe2x80x9cA Semiconductor Memory Device, and a Method for Read/Write the Semiconductor Memory Devicexe2x80x9d.
Article (1) describes a ferroelectric type NVDRAM as including memory cells each having two transistors and two capacitors (hereinafter this type of NVDRAM will be referred to as a two-transistor/cell NVDRAM). In the two-transistor/cell NVDRAM, a read/write operation is performed in a recall/store mode which involves polarization reversal of a ferroelectric thin film included in the memory cell. However, the two-transistor/cell NVDRAM cannot satisfy the required number of available polarization reversals (1015 to 1016 times) of the ferroelectric thin film in order to achieve 10-year continuous operation with a cycle of 100 nsec. This is because the possible number of the available polarization reversals of the ferroelectric thin film is limited to 1012 to 1013 times at maximum under the preset situation. Article (2) discusses the difference in fatigue of the ferroelectric thin film operated during a volatile (DRAM) operation without the polarization reversal and during a non-volatile (recall/store) operation. It is reported that the fatigue of the ferroelectric thin film is significantly small during the volatile operation compared with during the non-volatile operation. Article (3) describes an example of a non-volatile semiconductor memory device including word lines selected by row addresses and plate lines and sense amplifiers selected by column addresses, realizing low power consumption and a short cycle.
The above-mentioned ferroelectric type NVDRAMs use memory cells each incorporating a capacitor element which has a thin film made of a ferroelectric material with a crystal structure of a perovskite type, e.g., Y1 (the common name of a ferroelectric ceramic which is less fatigued by a rewrite operation (polarization reversal)), PZT, PLZT, PbTiO3, etc. Many kinds of ferroelectric materials have been vigorously developed in order to realize an ideal ferroelectric thin film which is not fatigued by the polarization reversal and which has a large remanent polarization and a large dielectric constant.
When an AC voltage is applied to the capacitor element including such a ferrroelectric thin film, the polarization of the ferroelectric thin film shows a hysteresis characteristic as shown in FIG. 11. As seen from FIG. 11, the polarization state of the ferroelectric material, which is at point A when it is not polarizaed, shifts to point B when a positive electric field is applied to the ferroelectric thin film. The polarization state of the ferroelectric thin film returns only to point C (instead of A) when the electric field is removed, thus resulting in a positive remanent polarization. This remanent polarization vanishes when a negative electric field is applied. By further increasing the intensity of the negative electric field, the polarization of the ferroelectric thin film is reversed so that the polarization state shifts to point D. The polarization state of the ferroelectric thin film returns only to point E when the electric field is again removed, thus resulting in a negative remanent polarization. Thus, by allowing the polarization of the ferroelectric thin film to be reversed so as to attain positive or negative remanent polarization, given data can be stored in a non-volatile manner. Moreover, by simply applying or removing a positive or negative electric field across the capacitor element, it can be ensured that the polarization state of the ferroelectric material of the capacitor element shifts only between points B and C or between points D and E, instead of having polarization reversals. Thus, given data can be stored in a volatile manner, as in the case of a usual DRAM.
The above-described ferroelectric NVDRAM has the advantage that, since the memory cells thereof can be constituted by a smaller number of elements than in the case of a combination type NVDRAM, the cell areas can be reduced, thereby providing for further integration of the device. The ferroelectric NVDRAM has another advantage of low power consumption.
Hereinafter, the structure and operation of an exemplary ferroelectric type NVDRAM, where two-transistor/cell memory cells are used, will be described. The two transistor/cell memory cell is known to be immune to possible variations in the fabrication process.
As shown in FIG. 12, this ferroelectric type NVDRAM includes a plurality of word lines WL and corresponding plate lines PT. The word lines WL are connected to a word line decoder 31. The plate lines PT are connected to a plate decoder 32. The NVDRAM also includes a plurality of pairs of bit lines, bit and bit-bar. Each pair of bit lines, bit and bit-bar, are connected to a sense amplifier 33. In FIG. 12, only one pair of bit lines, bit and bit-bar, and their corresponding sense amplifier 33 are shown.
A memory cell 34 is provided in each portion where one of the word lines WL and its corresponding plate line PT intersect a pair of bit lines, bit and bit-bar. In FIG. 12, only one memory cell 34 is shown. The memory cell 34 includes two capacitor elements C1 and C2 and two selection transistors Q1 and Q2. One of the terminals of the capacitor element C1 is connected to the bit line bit via the transistor Q1. One of the terminals of the capacitor element C2 is connected to the bit line bit-bar via the transistor Q2. The other terminals of the capacitor elements C1, and C2 are connected to the plate line PT. Gates of the transistors Q1 and Q2 are connected to the word line WL.
The above-described ferroelectric type NVDRAM operates in the following manner. The word line decoder 31 and the plate line decoder 32 select, respectively, one of the word lines WL and one of the plate lines PT, in accordance with an address signal input to an address buffer 35. Then, the memory cell 34 is accessed in a mode chosen in accordance with a control signalinput from an NE bar pin (an enable pin for switching to a non-volatile mode), a CE-bar bin (an enable pin for selecting a chip), an OE-bar pin (an output enable pin), a WE-bar pin (a write enable pin) to a control signal input buffer 36. Specifically, the access operation to the memory cell 34 is conducted in one of the following modes: in a DRAM mode (or a volatile mode) for accessing data stored in a volatile state, the NVDRAM is controlled by a DRAM mode timing control circuit 37; in a recall mode for reading and rewriting data stored in a non-volatile state, the NVDRAM is controlled by a recall mode timing control circuit 38; in a store mode for writing data stored in a non-volatile state, the NVDRAM is controlled by a store mode timing control circuit 39. The inputting and outputting of the data to be accessed are conducted through a data I/O interface 40.
Hereinafter, a write operation in the store mode, controlled by the store-mode timing control circuit 39, will be described with reference to FIGS. 11 through 14. As shown in FIG. 13, when data xe2x80x9c0xe2x80x9d is to be written, for example, a voltage of 0 V and a voltage of 5 V (e.g., supply voltage Vcc) are applied to, respectively, the bit line bit and the bit line bit-bar, and a voltage pulse which varies from 0 V and 5 V and back to 0 V is applied to the plate line PT while keeping the word line WL in an active state. As a result, the polarization state of the ferroelectric material of the capacitor element C1 shifts from point C or point E to point B, and then to point C as shown in FIG. 11. The polarization state of the ferroelectric material of the capacitor element C2 shifts from point D to point E and back to point D as shown in FIG. 11. Accordingly, after application of these voltages is stopped, the ferroelectric materials of the capacitor elements C1 and C2 have the remanent polarizations of points C and E, respectively, so that the data ""0 is stored in a nonvolatile manner.
When data xe2x80x9c1xe2x80x9d is to be written, as shown in FIG. 14, a voltage of 5 V and avoltage of 0 V are applied to, respectively, the bit line bit and the bit line bit-bar, in contrast to the voltages of 0 V and 5 V applied in the above-mentioned case of writing the data xe2x80x9c0xe2x80x9d. A voltage pulse which varies from 0 V to 5 V and back to 0 V is applied to the plate line PT while keeping the word line WL in an active state, whereby the ferroelectric materials of the capacitor elements C1 and C2 are caused to have the remanent polarization of points E and C, respectively, so that the data xe2x80x9c1xe2x80x9d is stored in a non-volatile manner. Alternatively, a plurality of voltage pulses which, for example, vary from 0 V to 5 V to 0 V to 5 V to 0 V applied to the plate line PT, whereby obtaining a more reliable store operation.
Next, a read operation of data in the recall mode, controlled by the recall mode timing control circuit 38, will be described with reference to FIGS. 11, 12, and 15. In this operation, as shown in FIG. 15, the pair of bit lines bit and bit-bar are precharged at a potential of 0 V and thereafter are placed in an open state. Then, a voltage which varies from 0 V to 5 V is applied to the plate line PT while keeping the word line WL in an active state. As a result, in the case where data xe2x80x9c0xe2x80x9d is stored in the memory cell 34, the polarization state of the ferroelectric material of the capacitor element C1 shifts from point C to point B as shown in FIG. 11, while the polarization state of the ferroelectric material of the capacitor element C2 shifts from point E to point B. Thus, the polarization of the ferroelectric material of the capacitor C2 is reversed, so that the potential of the bit line bit-bar, which is connected to the capacitor element C2, becomes higher than that of the bit line bit by hundreds of mV (milivolts). Accordingly, by sensing the difference between the potentials of the bit lines bit and bit-bar by the sense amplifier 33, the data stored in a non-volatile manner can be read out.
However, the polarization states of the capacitor elements C1 and C2 both shift to point B, so that the data which has been stored in a non-volatile manner is lost, resulting in a destructive read-out of the data. Accordingly, in the recall mode, the data which has been recalled can be rewritten in the NVDRAM by storing the data in a non-volatile manner. This can be conducted by, after the pair of bit lines bit and bit-bar are set at 0 V and 5 V by the sense amplifier 33, applying a voltage which varies from 0 V to 5 V and back to 0 V to the plate line PT in the same procedure as that for the store mode. In addition, by maintaining the plate line PT at 0 V after the above procedure, the potentials set for the pair of bit lines bit and bit-bar can be stored in the memory cell 34 as charges, thus realizing storage in a DRAM mode (to be described later). The potential difference between the pair of bit lines bit and bit-bar which is generated in the recall-mode is in proportion to the intensity of the remanent polarization of the ferroelectric material and in inverse proportion to the bit line capacitance. Accordingly, a larger potential difference can be obtained as the remanent polarization increases and as the bit line capacity decreases, thereby facilitating the detection by the sense amplifier 33.
An access operation to the data in the DRAM mode, controlled by the DRAM-mode timing control circuit 37, is achieved by the same procedure as that for a conventional DRAM except that a voltage of 0 V or 5 V is applied to the plate line PT. Thus, the polarization states of the ferroelectric materials of the capacitor element C1 and C2 shift only between point D and point E or between point B and point C (FIG. 11), neither of which results in any polarization reversal. Therefore, the read and write of the data stored in a volatile manner can be conducted by utilizing the charges stored in the capacitor elements C1 and C2 alone, as in the case of a conventional DRAM. Refresh operations of the data can also be conducted.
Although a ferroelectric type NVDRAM using two transistor/cell memory cells was described above, substantially the same principle applies to a ferroelectric NVDRAM including a cell having one transistor and one capacitor (hereinafter this type of NVDRAM will be referred to as a one-transistor/cell NVDRAM), as disclosed in Japanese Laid-Open Publication No. 4-176589, which is suitable for a high integration. In addition, the same principle may be applied to NVDRAMs in which addresses are multiplexed and a row address strobe signal RAS-bar and a column address strobe signal CAS-bar are used, as in a usual case of a DRAM. Multiplexing the addresses has the advantage of reducing the number of terminal pins for the addresses so as to obtain a higher integration of the device.
It is possible to operate the combination type NVDRAM and the ferroelectric type NVDRAM both described above in the store mode for writing data in a non-volatile manner and in the recall mode for reading the data stored in the non-volatile manner. However, the operational speed of the ferroelectric NVDRAM is slower than that of the combination type NVDRAM by the polarization reversal time.
Moreover, in the case of the combination type NVDRAM, available times to rewrite the EEPROM are limited to about one hundred thousand times, so that the EEPROM will soon reach the end of its lifetime if frequent rewrite operations are conducted. For this reason, in the case of the combination type NVDRAM, the EEPROM is prevented from undergoing an excessively large number of rewrite operations by ensuring that only the DRAM is accessed during normal operations. Accordingly, the data stored in the DRAM is saved in the EEPROM only immediately before the NVDRAM device is turned off, and all the data is recalled into the DRAM in a recall mode when the NVDRAM device is turned on the next time.
In the case of the ferroelectric type NVDRAM, the ferroelectric materials included in the capacitor elements C1 and C2 permit only a limited number of polarization reversals so that the recall/store operations are limited to about 1010 to 1012 times. Therefore, the lifetime of the memory cells 34 can expire in a few days by conducting successive accessing at a frequency of about 10 MHz. The number of polarization reversals of the ferroelecric material needs to be about 1015 or more in order to obtain the 10-year continuous non-volatile operation (recall/store operation). To this end, the number of the access operations which result in polarization reversals is minimized by ensuring that access operations in the DRAM mode, which do not result in any polarization reversal, are conducted during normal read/write operations and that the data in the memory cells 34 is stored in a non-volatile manner only before the NVDRAM device is turned off. In the ferroelectric type NVDRAM device, all the data stored in the non-volatile state is recalled to the data stored in the volatile state in the first read operation when the NVDRAM device is turned on the next time. In addition, since the access operation in the DRAM mode does not require varying a voltage level of the plate line PT, the access operation in the DRAM mode has the advantages of shortening the cycle and reducing power consumption.
FIGS. 16 and 17 show structures of conventional NVDRAMs called FRAM (or FeRAM). These FRAMs include row decoders to receive row addresses via row address latches and column decoders to receive column addresses via column address latches. The row and column decodersareprovided around memory cell arrays. A memory cell is selected in accordance with an input address. The memory cell is then accessed in accordance with a mode based on control signals CE-bar, WE-bar, and OE-bar input to a control circuit. Data to be written or read to/from the memory cell is input to a column decoder or output to the outside via I/O0 to I/O7 by an I/O latch and an I/O buffer circuit.
For example, in the case of a parallel FRAM having a small capacity (4 Kbit) shown in FIG. 16, higher address pin terminals A3 to A5 are assigned to a row address while lower address pin terminals A0 to A2 are assigned to a column address. However, in the case of a parallel FRAM having a large capacity (64 Kbit) shown in FIG. 17, higher address pin terminals A3 to A9 and A11 and A12 are assigned to a row address while lower address pin terminals A0 to A2 and A10 are assigned to a column address. In the case of the parallel FRAM having the large capacity (64 Kbit), the numbering of the address pin terminals is not serial.
Japanese Laid-Open Publication No. 7-13859 discloses volatile and non-volatile semiconductor memory devices having different capacities and widths of data bus and a controller for the devices. The controller reduces unnecessary power consumption due to penetration power, etc. as much as possible. However, this conventional technique suffers from a low operational speed of a semiconductor memory device since the number of operations of a non-volatile memory device is large.
As NVDRAMs, the combination type NVDRAM including a DRAM as a volatile data storage means and an EEPROM as a non-volatile data storage means, and the ferroelectric type NVDRAM including a memory cell having volatile and non-volatile storage capabilities are described above. In either type NVDRAM, if only the non-volatile (recall/store) operations are performed continuously, a 10-year operation cannot be guaranteed.
Further, in the conventional NVDRAMs, there are problems in that the recall/store operations are slow; data may be not protected; and the polarization is fatigued.
In the NVDRAMs, data stored in the non-volatile state is transformed to data in the volatile state in the recall operation while data stored in the volatile state is transformed to data in the non-volatile state in the store operation. In the non-volatile (recall/store) operations, the polarization reversal of the ferroelectric material occurs, leading to the longer operation time as compared with during the volatile (DRAM) operation. Therefore, when the NVDRAM is operated in the non-volatile (recall/store) operations, there is a problem in that the read and write speeds are slow. There is also a problem in that a system noise or the like upon power-on or power-off may cause an erroneous rewrite operation which unexpectedly changes non-volatile data. The ferroelectric materials are fatigued after data rewrite operations utilizing the polarization reversal are repeated about 1010 to 1012 times. If the device is operated at a cycle of 100 nsec, a 10-year successive access operations to the memory cell cannot be guaranteed.
Further, when the NVDRAM has a large memory capacity, the refresh operation of the conventional volatile DRAMs increases power consumption.
The above-described problems are common to the case where the NVDRAM is implemented as a memory cell device solely; the case where the NVDRAM is implemented as a memory module or register for a single chip microcomputer (MPU or CPU); the case where the NVDRAM is incorporated into a system in the form of a card, and the like.
In the store operation of the combination type NVDRAM, a Fowler-Nordheim tunnel current flows through the EEPROM, requiring an additional time as compared with the volatile (DRAM) operation. Therefore, the write speed is slow. Moreover, the number of rewrite operations is limited to about one hundred thousand times for the EEPROM. The EEPROM will soon reach the end of its lifetime if frequent rewrite operation are conducted. A 10-year successive access operation thus cannot be guaranteed.
According to one aspect of the present invention, a non-volatile semiconductor memory device includes a memory cell array including memory cells capable of storing data in a volatile state and in a non-volatile state. The memory cells are arranged in a matrix. The non-volatile semiconductor memory device further includes a row address determination circuit for determining whether a row address in a current cycle is the same as a row address in a previous cycle when an activating signal for activating a read cycle for reading data stored in the memory cell or a write cycle for writing data into the memory cell is received. When the row address in the current cycle is different from the row address in the previous cycle, a store operation is performed with respect to the memory cell selected in the previous cycle, where volatile data is stored in the memory cell as non-volatile data; thereafter, a recall operation is performed where non-volatile data stored in the memory cell selected in the current cycle is transformed into volatile data; and the resulting volatile data is stored in the memory cell and a sense amplifier connected to a corresponding column address, or at least one of the sense amplifier and a latch circuit. When the row address in the current cycle is the same as the row address in the previous cycle, a read or write operation is performed with respect to volatile data stored in the memory cell, or the sense amplifier or latch circuit connected to a corresponding column address, but non-volatile data is not read from or written to the memory cell.
In one embodiment of this invention, when the read operation is performed immediately after power-on, a reset signal changing upon power-on or a reset signal inside the device is used in the recall operation.
In one embodiment of this invention, upon power-off or reset, a reset signal changing upon power-off or reset, or a reset signal inside the device is used in the store operation.
In one embodiment of this invention, the column address is assigned to lower address pin terminals, and the row address is assigned to higher address pin terminals; and an external address is input.
In one embodiment of this invention, each memory cell includes a combination of a DRAM as a volatile data storage means and an EEPROM as a non-volatile data storage means.
In one embodiment of this invention, each memory cell is made of a ferroelecric material; and data is stored in a non-volatile state in each memory cell using polarization reversal and data is stored in a volatile state in each memory cell without using the polarization reversal.
In one embodiment of this invention, the device is controlled in such a way that a driving signal for driving a word line connected to the memory cell is in the enable state during a period of time from the start of the recall operation to the end of the store operation.
In one embodiment of this invention, the device is controlled in such a way that a driving signal for driving a word line connected to each memory cell is in the enable state only during the recall operation and the store operation.
In one embodiment of this invention, the store operation is initiated after the recall operation is completed.
In one embodiment of this invention, the read or write cycle is performed in synchronization with an external synchronization signal.
In one embodiment of this invention, the non-volatile semiconductor memory device further includes a circuit for generating a WAIT signal, wherein the WAIT signal is inactive during the recall operation and the store operation.
In one embodiment of this invention, the non-volatile semiconductor memory device further includes a latch circuit for latching a row address at least during the read cycle and the write cycle.
According to another aspect of the present invention, a system LSI includes the non-volatile semiconductor memory device.
Hereinafter, functions of the present invention will be descried.
As NVDRAMS, the combination type NVDRAM including a DRAM as a volatile data storage means and an EEPROM as a non-volatile data storage means, and the ferroelectric type NVDRAM including a memory cell having volatile and non-volatile storage capabilities are described above. In either type NVDRAM, a memory cell has volatile and non-volatile data storage capabilities.
According to the present invention, in the NVDRAMs, when an activation signal is input f or activating a read cycle of reading data stored in a memory cell or a write cycle of writing data into the memory cell, the row address determination circuit determines whether a row address (word line address) input in a current read/write cycle is the same as a row address input in the previous read/write cycle.
When the row addresses differ from each other between the current and previous cycles, the store operation Is performed for a memory cell which has been selected by a row address in the previous cycle, where data stored in the volatile state is transformed into data stored in the non-volatile state. Thereafter, the recall operation is performed for a memory cell which is selected by a row address in the current cycle, where data stored in the memory cell in the non-volatile state is transformed into data in the volatile state for the purpose of reading. The obtained data stored in the memory cell in the volatile state is also stored in a sense amplifier connected to a column address corresponding to the memory cell. In either case of the ferroelectric type NVDRAM or the combination type NVDRAM, data may be stored only in the sense amplifier. Alternatively, data may be transferred and stored in the latch circuit rather than the sense amplifier. Alternatively, data may be stored in both the sense amplifier and the latch circuit. Data is stored in the volatile state in this way, thereby making it possible to reduce consumption of the polarization reversal. In the case of the combination type NVDRAM, the number of rewrite operations can be reduced. Further, the sense amplifier or the latch circuit includes a circuit which compensates a leakage current. Therefore, when data is stored in the volatile state in the sense amplifier or the latch circuit, the refresh operation which is performed in the conventional volatile DRAM is not required, thereby reducing power consumption.
When a row address selected in the current cycle is the same as a row address selected in the previous cycle, the read/write operations are performed with respect to data stored in the volatile state in a memory cell or a sense amplifier or latch circuit connected to a column address corresponding to the memory cell. In this case, the data stored in the non-volatile state is not read from or written to the memory cell. When the write operation is performed with respect to external data, if this operation is performed on the data stored in the volatile state in the sense amplifier or the latch circuit, the refresh operation which is performed in the conventional volatile DRAM is not required, thereby reducing power consumption. Further, when a column address selected in the current cycle is the same as a column address selected in the previous cycle, an operation requiring the polarization reversal (non-volatile operation) is not performed, so that the read speed is increased. Moreover, an SRAM having a small capacity can be omitted, thereby reducing cost. In the case of the combination type NVDRAM, the rewrite operation which causes the tunnel current is not performed, thereby increasing the rewrite speed.
When a device or system is turned on (powered on), a previous address is not specified. In this case, the recall operation can be controlled utilizing a reset signal which changes upon power-on (e.g., a system reset signal, or a reset signal within the device). Upon power-off, or when the system is reset, the store operation can be performed utilizing a reset signal which changes upon power-off or system reset (e.g., a system reset signal, or a reset signal within the device). At that point, by utilizing the system reset signal which inactivates the whole system, an erroneous write operation due to noise can be prevented.
For example, as will be described in the Examples, when the CE-bar (or the RAS-bar in the case of the address multiplex type) is activated (active state xe2x80x9cLOWxe2x80x9d) after power-on, the recall operation is performed with respect to a memory cell provided along a word line selected by a current row address. The resulting data obtained is then stored in the memory cell and a sense amplifier connected to the corresponding column address. Alternatively, the data may be stored in a latch circuit, or both in the sense amplifier and in the latch circuit. When a row address in a current cycle is the same as a row address in the previous cycle (a CEin-bar signal keeps the active (xe2x80x9cLOWxe2x80x9d) state), the read/write operation is performed with respect to data stored in the sense amplifier or the latch circuit connected to the corresponding column address. When a row address in a current cycle is different from a row address in the previous cycle (a CEin-bar signal keeps the inactive (xe2x80x9cHIGHxe2x80x9d) state), the store operation is performed with respect to data stored in the volatile state in the sense amplifier or the latch circuit is stored as data in the non-volatile state.
Whereas in the conventional NVDRAMs the recall operation is performed with respect to a memory cell using an NE (Non-Volatile Enable)-bar signal which enables a non-volatile operation, the recall operation is performed with respect to a memory cell selected by a row address when the CE-bar signal (or the RAS-bar signal in the case of the address multiplex type) is active (xe2x80x9cLOWxe2x80x9d) so that data is stored in the volatile state in a sense amplifier or the like which does not require the refresh operation and the other memory cells are used in the non-volatile manner which does not require the refresh operation. Thus, the refresh operation is not required for data stored in the volatile state, thereby reducing unnecessary power consumption. The CE-bar signal (or the RAS-bar signal) also serves as the NE-bar signal, so that a user does not recognize the NE-bar signal. Therefore, the NVDRAM of the present invention can be used in a way similar to that of the conventional volatile DRAMs, analogous SRAMs, or SRAMS, thereby obtaining satisfactory usability.
Further, the column address may be assigned to lower address pin terminals (A0-) and the row address may be assigned to higher address pin terminals. The address pin terminals are selected in ascending order from the lower address. This allows satisfactory compatibility with a system CPU and a control circuit, thereby obtaining satisfactory usability.
Typically, the number of column addresses having the same row address is about 100 to about 1000. The present invention is particularly effective to the case where data stored at serial addresses (e.g., file memory) are used. For example, the number of available polarization reversals (1012 to 1013) is multiplied by the number of the column addresses (100 to 1000), for exampe, 1000. The result is 1015 to 1016 which corresponds to the number of rewrite operations. The 10-year continuous operation (with a cycle of 100 nsec) can be achieved. In this case, the present invention is particularly effective when the column address is assigned to lower address pin terminals (A0-) and the row address is assigned to higher address pin terminals.
The NVDRAM of the present invention may be controlled in such a way that the driving signal (row decoder control signal) for driving a word line connected to a memory cell is in. the enable state (xe2x80x9cHIGHxe2x80x9d) during the period of time from the start of the recall operation to the end of the store operation. In this case, the circuit for generating the control signal can be simplified. Alternatively, the NVDRAM of the present invention may be controlled in such a way that the driving signal (row decoder control signal) for driving a word line connected to a memory cell is in the enable state only during the recall operation and the store operation. In this case, the driving signal is in the form of pulses, thereby reducing power consumption.
When the store operation is controlled upon power-off using a reset signal which changes upon power-off or system reset (e.g., a system reset signal or a reset signal within a device), the store operation can be initiated after the recall operation is completely ended so that data is not destroyed during the recall operation. An example of the reset signal within a device is a POWERGOOD signal.
Further, as in a synchronous system or a burst system, the read/write operations may be performed in synchronization with a synchronization signal from an external control system or the like. In this case, the synchronization signal can be utilized for the delay circuit so that the delay circuit can be simplified, thereby reducing variation in delay time.
A circuit for generating a WAIT signal may be provided so that the CPU or the control circuit can be in the inactive state (xe2x80x9cLOWxe2x80x9d) during the recall/store operations to keep the read/write states during the recall/store operations.
A row address may be latched when the CE-bar signal for controlling the read/write cycles is in the active state (active=xe2x80x9cLOWxe2x80x9d). This allows further prevention of an erroneous write operation due to noise or the like. The latching of a row address in such a period of time leads to prevention of an erroneous operation due to noise even when an input row address fluctuates due to noise, because the row address has already been latched inside the device.
Thus, the invention described herein makes possible the advantages of providing an NVDRAM and a system LSI which have high-speed recall/store operations and reduced power consumption and in which a 10-year continuous operation can be guaranteed.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.